Method of heating and sintering



July 23, 1963 3,098,738

J. D. MADARAS METHOD OF HEATING AND SINTERING Filed Aug. 50, 1954 IN VENTOR. Jul/U5 0 MHOARHS ATTORNEYS 3,098,733 METHOD ()1? HEATING ANDSINTERING Julius D. Madaras, Longview, Mich, assignor to GasIncorporated, Detroit, Mich, a corporation of Michigan Filed Aug. 30,1954, Ser. No. 452,913 4 Claims. (Cl. 75-34) My invention relates to theheating and sintering of mineral materials, principally ores, that aremined or produced in such a physical state that they must be fused intolarger masses for further handling, heated for further chemicaltreatment or otherwise subjected to an oxidizing roast. This applicationis a continuation in part of my copending application Serial No.125,932, filed November 7, 1949, now abandoned. By the application of mymethod to these well known steps great savings in time, fuel andhandling costs can be achieved. Like- Wise certain physical propertiesof mineral materials can be developed leaving these materials in such acondition that further processing can be performed in Ways which, thoughwell known, have not been economically practical.

My method is limited only in its application to those materials that canbe heated in the presence of burning carbonaceous, hydrocarbon, sulphurand other fuels with air or in some cases oxygen or oxygen enriched airis described.

The material to be heated is crushed, if necessary, to a size that willpermit relatively free passage to air and other gases through a bed ofthe material or if originally in a more finely divided state, thematerial is agglomerated to a similar gross particle size in theapproximate range of inch to 2 inches or more. The material to be heatedis mixed with a fuel material such as coke breeze, powdered coal,hydrocarbon fuel or powdered sulphur in such a manner that the fuelmaterial is more or less equally distributed throughout the mixture. Inthe case of sulfide ores a large part or all of the necessary fuel forheating or sintering is naturally distributed in the ore. It isimportant that the fuel material and all other charged materials befixed in position in the mixture in such a manner that they shall not beseparated or disintegrated (in the case of glomerules) during thesubsequent steps described below.

To achieve adequate fixation in place of the mixed materials asrequired, I use a liquid binder which may be water or water containingcarbohydrate such as wood pulp sludge Waste or other pastes. Any binderswhich are used in pelletizing generally are adequate. I have also usedliquid hydrocarbons as a binder. In such cases the fuel content of thebinder must be accounted for as described below as replacing all or partof the fuel materials which must be added to achieve desired heating.

The mixing of crushed ore, such as raw limestone, with the calculatedamount of fuel and binder may be carried out in any suitable mixingdevice, not shown. The amount of binder to be used is best described asthe minimum amount needed to wet the charged material, limestone in thiscase, and the finely divided carbonaceous fuel to such an extent thatsubstantially all the fuel is evenly distributed around and adheres tothe surfaces of the larger limestone ore particles. This minimum amountof binder will be diiferent for difierent binders and different mineralsbut for limestone and most iron ores water varies between 4% and 6% ofthe weight of ore. For reasons explained below Water or water producingfuel binders should be held to the The mixture is then charged into asuitably designed pressure retort.

In the drawing is illustrated a type of apparatus suitable for carryingout my process. As shown, 10 is the outer casing of a pressure retortwhich is suitably lined with a refractory lining =11, which in manycases may be 3,698,738 Patented July 23, 1963 omitted. .A gas-tightcover 12 is secured to the retort by means of brackets :13 and bolts 14,and may be removed when charging or removing the mixture from theretort. An inlet conduit 15 provided with a control valve 16 isconnected to a source of compressed air or gas, while another conduit 17serves as an outlet and is controlled by a valve 18. The control valve18 serves to connect the outlet conduit to a vacuum tank 19, and may beso operated as to by-pass the vacuum tank and connect the out-letdirectly to an exhaust conduit 20 when desired. This valve may be of anydesired construction, and may be manually or mechanically operated. Itmay also be automatically operated and controlled by the difference inpressure between the retort and the vacuum tank or the exhaust conduit.

For purposes of illustration, the retort is shown as charged with amixture of limestone 21 and coke 22, although other materials may beprocessed by my method as will hereinafter be set forth.

After the retort is charged, hot compressed air is admitted throughvalve 16 while valve 18 remains closed. The hot air ignites thecarbonaceous material in the charge, and after a suitable length of timevalve 16 is closed to shut off the air and valve 18 is opened to permitthe escape of the gaseous products of combustion through exhaust conduit20. Valve 18 is then closed and more hot air is admitted into theretort, and the cycle is repeated. By alternately opening and closingthe inlet and outlet valves, air is repeatedly injected into the chargeand the products of combustion along with the other gases and vapors areexhausted. In this Way the entire charge in the retort is heated and itschemical composition is changed. In some cases only one of the valvesmay be operated while the other is kept open, still providing suflicientpulsation of the air through the mass to accomplish etficient heatingand reaction of the charge. The nature and advantages of the inventionwill be made more clear by the following discussion.

Hot air burning with coke or other carbonaceous material gives a hightemperature and the heat .is conducted rapidly toward the inside of thelumps of the charge. The heat gradient between the surface and theinside of the lumps is greater than can be provided if unheated air isused. The high temperature of air and coke at the place of combustionhelps in igniting the coke faster and burning it more completely,however unheated air may also be used without departing from myinvention.

As the lumps of limestone are heated, the carbon dioxide in gaseous formleaves the limestone leaving lumps of lime behind. As the combustiongradually proceeds downward, or upward if such a heating arrangement isdesired, it will leave behind a hot layer in which the burning has beencompleted. Upon injecting hot air into the charge part of the air goesaround the lumps or particles of the charge, but as the pressure buildsup the hot air penetrates the pores of the lumps. When the pressure isreleased upon opening the exhaust valve, the air will burn as it passesthrough the hot unburned layers of coke while escaping from the retort.

Repeatedly injecting hot air into the charge eliminates the channelingof air through the mass, burns the combustible more economically,provides higher temperature and heats the charge more uniformly than dothe methods commonly used,

The temperature of the hot air that is admitted through the inlet valveis further increased when it contacts the surface of the hot lumps ofthe charge. As the pressure builds up within the retort the air isforced into the pores of the lumps and carries heat into the lumps at ahigher rate than if the heat were carried to the inside of the lumps byconduction alone. This is especially true since the outside heated partof the lumps is made more porous and less heat conducting after theouter surface has become calcined. Without the pulsating pressureprovided by my method, the calcining of the lumps becomes more diflicultas the outer portion becomes progressively more porous and less heatconducting. This difiic-ulty is increased as the CO begins to forminside the lumps, since in escaping from the lumps the CO tends toconteract the effect of heat conduction and also tends to cool the outersurface of the lumps. In contrast to this, when the pressure ispulsated, hot air is forced into the lumps through the porous outersurface. The hot air that is forced into the lumps carries its heatdirectly to the interior of the lumps, and also serves to mechanicallydrive some of the heat from the outer surface directly into the lumps.

In addition, the mechanical action of forcing the air into the lumpscreates pores much more rapidly than they would otherwise be created bythe action of the CO trying to escape from the lumps. With my pulsatingmethod the calcining of the limestone, that is to say, the removal offrom the lumps, is accomplished partially by the heat and partially bythe mechanical force of the pressure differential.

An added advantage will be derived from calcining limestone by my methodWherever the exhausted C0 is to be further used for chemical purposes.Thus the CO Will be more concentrated in the exhaust gas due to moreefiicient use of air which results in less air being used and thereforeless nitrogen being present in the gases. This effect may be furtherenhanced by enriching the air with oxygen or if desirable, by using alloxygen. In the latter instance there is no need to preheat the oxygen.

A further advantage of my method is that the temperature of calciningmay be lowered to a point where there is little tendency for the lime tofuse or ilux with the earthy impurities such as alumina or silica ormaterials in the retort wall insulation. Furthermore, my methodovercomes the objection to ordinary methods of calcining wherein aconsiderable part of the CaCO in the center of the lumps is not calcinedand must be removed at considerable expense.

Another example illustrating the invention is that of burning orcalcining cement. A predetermined mixture of ingredients for making aparticular type of cement is ground and mixed together and pelletized toform small pellets or glomerules in a manner as described below. Theglomerules then mixed with a proper amount of carbonaceous material, theamount of which is determined by the ultimate temperature desired. Thetemperatures in burning lime or cement as well as the ingredients of anyparticular cement are known to anyone versed in the art, and need not bespecified here. The mixture is charged into the retort and burned byinjecting air, preferably hot air or overventilated combustion productssubstantially in the manner previously described. If unheated air isused, or if necessary when using heated air, carbonaceous material maybe spread over the top of the charge and ignited before closing theretort. Part of the combustible may also be powdered and mixed with thecement ingredients before pelletizing, and the balance of thecombustible mixed with the charge and burned in the above describedmanner. After the burning is completed the charge may be cooled byinjecting cold air into the retort.

Still another example consists of pretreating, heating and reducingmetallic oxides, iron ore for instance. Crushed iron ore or mill scale,which may be lump, fairly fine or pelletized or agglomerated from fines,is mixed with the proper amount of carbonaceous material and a smallamount of water as described below and charged into the retort. Theamount of carbonaceous material, coke breeze for instance, will depend agreat deal upon the heating value of the combustible and the temperature to be obtained. In general, in the case of iron ore,approximately 2.5% to 3% coke breeze is required for heating purposes.When fusion of the mass is required, the amount may be increased to 4%or even 5%. After the vessel is charged with the mixture, hot air isadmitted into the retort where it ignites and burns the combustible. Aspreviously described, repeatedly injecting the air and exhausting theproducts of combustion will eliminate channeling and will help inheating the inside of the lumps thoroughly, uniformly and efiiciently.Moreover, the hot air will treat the ore by burning out all of thesulphur, and will leave the ore porous for the subsequent reduction ofthe ore by reducing gas.

When the iron ore is intended to be reduced to iron, it is preferable tokeep the average firing temperature of the mass below the sintering,clinkering or fusing point so that the reduction of the ore withreducing gases is not made more difficult. If the fusion point isreached or exceeded, iron silicate will form and will materially hinderthe reduction of the ore. Similarly, other oxides will fuse at thesintering temperature of the ore, and calcium carbonate or similarmaterial will act as flux to lower the fusion point and thereby makesubsequent reduction of the ore more diflicult.

My method, however, may be used for sintering or even fusing the entiremass of iron ore. Such fused or over sintered ore is much more densethan ordinary ore sinter, and may advantageously be used as lump ironoxide for decarbonizing the bath of steel in the open hearth or theelectric furnace since the heavy, dense lumps readily sink through themolten slag covering the molten steel bath.

It should be noted that mill scale may also be sintered by my process,and fused so that it forms heavy lumps. This is particularly valuabledue to the fact that mill scale is relatively pure iron oxide, andcontains no slag material or only a very small amount of it. Aftertreatment in the retort, the material may be put through a roller orpress to shape it to the desired size.

If it is desired to bring about more complete melting or fusion of themass after it is heated, reducing gas such as hydrogen or carbonmonoxide is repeatedly injected into the hot charge so that it will bepartially reduced. Hot air is then repeatedly injected to reoxidize thepartially reduced ore, thereby producing an additional amount of heatwithin the charge sufficient to fuse the ore. The desired temperaturecan be reached by regulating the amount of combustible and thetemperature of the air, as well as the extent of reduction of the orewhich in turn is determined largely by the amount of reducing gas used.A more comprehensive description of the above outlined process iscontained in my copending applications bearing Serial Numbers 125,931and 125,933.

The mass of mixture charged into the retort has considerable resistanceto the flow of gas therethrough. This resistance is determined mostly bythe size of lumps, the fine particles in the mass, the moisture and thedepth of the charge. If desired, the density of the mass may beincreased at the start of heating by building up a high pressure of airwithin the retort. The air pressure on top of the charge can be easilyregulated to provide the desired force to press down on the mass.Similarly, when greater porosity of the charge is desired the airpressure at the beginning of heating can be low until the air goingthrough the mass has worked a certain porosity to lower the resistanceto the flow of air or gas. The pressure may then be increased graduallyto control the porosity of the heated or sintered mass. Also, pulsationmay be started only a few minutes after the combustion of the top layerof the charge has been completed. The gas or air pressure as well as thevalve operation that provides the pulsation may be mechanicallycontrolled and varied at will, depending upon the results to beobtained.

Iron ore may also be mixed withvlime or limestone and carbonaceouscombustible before being charged into the retort. This charge will betreated and heated as above described. However separating the CO fromone pound of lime requires 1380 B.t.u. of heat for which fuel has to beadded in addition to the fuel needed for raising the temperature of themass. Since only about 650 B.t.u. is needed to preheat a pound of iron,the iron ore would be melted or fused too much for subsequent bestreduction in solid form by reducing gas. Therefore, it is desirable insome instances to change the retort with alternate layers of iron oremixed with its proper share of carbonaceous fuel and layers of limestonemixed with its proper share of fuel. The amount of fuel mixed with thelimestone amounts to approximately twice to three times the amountrequired to preheat the iron ore.

The above cited applications of my method all recite the use of crushedmineral or ore materials. In each case a binder must be used for thefollowing reasons. As described the charged mixture is subjected topassage of gases through the charge, pulsated or not as described. Thisgas flow, if admitted through the charge at a practical economical rate,will tend to segregate the fuel material from the ore material resultingin poor heating in pants of the charge and over sintering in otherparts. The wetting of the mixture material in the mixing step sufiicientto prevent this segregation in later steps is described above.

=In addition to the use of air or oxygen in the manner described abovethe inlet air may be burned with a desired amount of fuel in the top ofor ahead of the vessel to be passedthrough the charge at a predeterminedtemperature and containing sufficient excess oxygen to burn with thefuel in the charge.

In the described cases the best heating is done when the combustion airis pulsated into the mixture charge. By pulsating the pressure fresh airis brought into direct contact with the fuel in a narrow burning zoneextending across the vessel as it progresses through the charge in thedirection of air flow. Where the burning is restricted to a narrow zoneat any particular time there is a desirable concentration of thecombustion reaction. This permits or causes more intense local heat atthe burning face with great savings of fuel to accomplish a given degreeof heating or sintering as compared with achieving similar minimumheating throughout the charge by burning with air admitted with a steadyflow.

A very uniform heating or sintering may be accomplished very quickly andefficiently by my method. With a uniform distribution of fuel andpulsated air all parts of the charge are heated equally and as quicklyas the air is admitted in any cross section of the vessel. This isparticularly true of sintering where a minimum fusion temperature isrequired in all parts of the charge. By using my method no part of thecharge need be oversintered to insure sintering of all parts of thecharge.

By taking advantage of this narrow zone of concentrated combustion loosemineral materials may be very loosely sintered or fused only at thecontact points between the ore particles or where particles of fuel werepresent. This type of sintering is very easily controlled by measurementof the fuel and inlet air temperature for any type of ore. With thissintering the internal structure of the ore particles is changed verylittle while the entire vessel charge is fused into a semi solid massthat can withstand severe handling in later treatment. This treatment isparticularly adapted to the sintering of agglomerated finely ground oressuch as taconite and other flotation concentrates. In this case theglomerules need to be fixed in a permanent condition so they withstandsevere handling. It is desirable to fuse the interior of the glomerulesor pellet-s as little as possible to permit easier chemical treatmentsubsequently. 'By my method it is very easy to give these pellets atough sintered surface of desired thickness leaving the interiorunchanged.

Using taconite concentrates as an example of the use of my process onvery fine minerals, I will describe the treatment to sinter them into ausable form for further smelting and the treatment to produce metallicsponge iron in .a single vessel directly. Taconite is a flotationconcentrate of finely divided iron ore sized approximately 200 to 400mesh. My method of treating is as follows: Starting with dry taconite Iplace it in a standard drum pelletizer and adding water I work the oreinto pellets or glomerules sized in a range from 4 inch to 1", ideally/2 inch; larger pellets do not sinter as easily being too much largerthan average and smaller pellets obstruct gas flow in a later step. Theamount of water is critical at this step as too much water destroys thepellets and will interfere later as described below. By using an organicbinder such as beer, pulp waste liquor or equivalent the necessary watermay be decreased. After separating oifsize pellets I add 2% to 2 /2% byweight of coke breeze to the ore pellets. This carbon should be crushedsmall with respect to the size of pellets, less than 20 mesh, althoughcoarser fuel has been used successfully. The fuel and ore are furthermixed in the drum mixer until all the carbon is impressed into thesurface of the damp pellets or adheres to the pellets in a substantiallyuniform distribution. These pellets or glomerules are now charged into avessel described in the drawing. The pellets made in the above describedmanner have suflicient strength to Withstand the handling involved intransferring them from the mixer to the pressure vessel.

After the vessel 10 is charged with the pelletized ore mixture 21, 22,the cover is closed and the vessel made gas tight, preheated air or overventilated combustion products are admitted through valve 16, ifnecessary incandescent charcoal on top of the charge may be used tostart the sintering. Combustion products and steam are exhausted throughvalve 18. In the same manner as described above the sintering operationis continued to completion.

As the burning zone progresses downward through the charge the waterused in making the pellets is vaporized when the combustion faceapproaches any particular part of the ore. This water vapor joins thestream of combustion products and moves toward the exhaust. By thismethod gases pulsated through solid mass are made to give up their heatto the solid most effectively. There fore, after leaving the burningzone, these combustion products cools very rapidly by heat exchange withthe ore at lower levels and the water vapor in the stream condenses. Ascan readily be seen, this water accumulates in front of the burning zoneand moves downward with it. If excessive water is used in the originalmixture of ore and fuel this accumulated water will wash much of thefuel down to the bottom of the charge and the heat distribution will bepoor in the lower part of the charge with over sintering at the verybottom. This problem is even more important with taconite and other finematerials that in pellet form have only wet strength and disintegratereadily in an excess of water. -I have noted that these accumulations ofwater can disintegrate the taconite pellets and completely cut off gasflow as described. Using Mesabi taconite pelletized with the minimumamount of water as a binder a charge depth of 4 to 5 feet is the maximumdepth of charge that will permit no excessive accumulation of water whenfired by this method. Certain pyritic ores require so much water inpelletizing that less than one foot of charge depth can be toleratedwithout disrupting the pellets and interfering with the gas flow.

It can be seen from the above paragraph that the exhaust temperature isnear 212 F. as shown by the presence of accumulated water. The outlettemperature remains steady in this range until a matter of momentsbefore completion of the heating. In a 5 foot depth of ore charge heatedin 20 minutes, the finish temperature swings from a steady 200 F. tofinish temperature of 1700 F. in less than one minute indicates theconcentration of heat exchange and combustion zones that can beaccomplished by my method of heating using a pulsating gas flow.

A modification of the above described method of treating taconites canbe used to prevent accummulations of water that affect treatment of thecharge. By reversing the inlet as shown in the drawing from top tobottom and the outlet from bottom to top, or in some cases not using thevessel cover and exhausting to the atmosphere directly, I can apply thesame method and avoid accumulation of water. In the vessel I place thepelletized taconite fuel mixture to a depth of not more than one footand ignite the fuel with preheated air. Immediately the temperature atthe top of the charge will rise considerably above 212 F. and preventaccumulation of water at the top with excess water exhausting as vapor.By feeding in new charge material at the rate burning progresses up thevessel as measured by a constant temperature at the charge surface inthe vessel, the vessel can be completely filled. By the time the vesselis filled the sintering is almost complete and the vessel may be closedas described and the hot charge subjected to further treatment. A shortperiod of pulsating air through the charge will remove last traces ofcarbon or sulphur present in the ore.

I have used liquid fuels such as kerosene as a binder in making pelletssuitable for sintering by my method. The procedure is the same exceptallowance must be made for the heat value of the fuel and an equivalentreduction made in the amount of solid fuel added. In the case of pyritesall of the carbon or other fuel may be replaced and the combustion mayhave to be moderated by using cooler air or by diluting the air withadditional steam to keep from over sintering. In cases where the sulfurdioxide concentrations warrant it the exhaust products may be passedthrough sulfur recovery apparatus.

In all the above cited cases an intermediate result can be a charge ofoxide ore heated to any desired temperature in a pressure vessel. Anadditional step in this invention would be to reduce this ore in placewithout further handling in the same vessel by the cyclic injection andexhausting of reducing gas through the same valves, reducing to anydegree, desired or possible, under the chemical conditions available forgaseous reduction at less than fusion temperature.

I claim:

1. A method of heating sintering and reducing a mass of divided ironoxide solids comprising the steps of homogeneously mixing carbonaceousfuel with the iron oxide solid using an amount of liquid binder toprovide substantial fixation in place of all elements of the mixtureduring the subsequent steps, charging the mixture into a pressurevessel, closing said vessel to make it gas tight, injecting air to burnwith the fuel thereby raising the pressure above atmospheric, exhaustingthe products from the charge to lower the pressure and adding the stepof cyclicly injecting reducing gas and exhausting products from thecharge to reach the desired state of reduction in the charge and therebyproduce sponge iron.

2. Process which comprises mixing finely divided iron ore about 200 to400 mesh with a limited amount of water and an organic binder to formpellets of a size about A" to 1", further mixing with added coke breezepellets into a pressure vessel having a gas inlet and a gas outlet,closing the vessel to make it gas-tight, introducing air into the gasinlet to burn with the carbonaceous ma terial thereby producing the heatto raise the temperature of said ore pellets and also raising thepressure above atmospheric, exhausting the combustion products throughsaid gas outlet to lower the pressure, and repeating the cycle until theore is heated to a final temperature about 1700 F.

3. Process which comprises mixing finely divided iron ore about 200 to400 mesh with a limited amount of water and an organic binder to formpellets of a size about A" to 1", further mixing with added coke breezeto coat the surface of the pellets, charging said coated pellets into apressure vessel having a gas inlet and a gas outlet, closing the vesselto make it gas-tight, introducing air into the gas inlet to burn withthe carbonaceous material thereby producing the heat to raise thetemperature of said ore pellets and also raising the pressure aboveatmospheric, exhausting the combustion products through said gas outletto lower the pressure, repeating the cycle until the ore is heated to afinal temperature about 1700 F., and further subjecting to cyclicinjection of reducing gas followed by exhaustion of the reactionproducts until the ore is reduced to sponge iron.

4. Process which comprises mixing finely divided iron ore with a limitedamount of Water and a binder to form pellets, further mixing withaddedcarbonaceous material to coat the surface 'of the pellets, chargingsaid coated pellets into a pressure-vessel having a gas inlet and a gasoutlet, introducing air into the gas inlet to burn with the carbonaceousmaterial thereby, producing the heat to raise the temperature of saidore pellets and also raising the pressure above atmospheric,- exhaustingthe combustion products through said gas outlet to lower the pressure,repeating the cycle untilthe pellets reach a desired final temperatureat least about, 1700 PI, and further subjecting tocyclic injectionofreducing gas followed by exhaustion of the reaction products until theore is reduced to sponge iron.

References Cited in the file of this patent UNITED STATES PATENTS263,310 Browne Aug. 29, 1882 420,371 Willcox Jan. 28, 1890 434,830 JoyAug. 19, 1890 685,064 Schubert Oct. 22, 1901 2,090,868 Hyde Aug. 24,1937 2,243,110 Madaras May 27, 1941 2,450,343 Howard Sept. 28, 19482,468,738 Durfee et al. May 3, 1949 2,548,876 De Jahn Apr. 17, 19512,666,632 Culver et al Jan. 19, 1954 FOREIGN PATENTS 3,328 Great Britain1885 24,803 Great Britain 1898

2. PROCESS WHICH COMPRISES MIXING FINELY DIVIDED IRON ORE ABOUT 200 TO400 MESH WITH A LIMITED AMOUNT OF WATER AND AN ORGANIC BINDER TO FORMPELLETS OF A SIZE ABOUT 1/4" TO 1", FURTHER MIXING WITH ADDED COKEBREEZE TO COAT THE SURFACE OF THE PELLETS, CHARGING SAID COATED PELLETSINTO A PRESSURE VESSEL HAVING A GAS INLET AND A GAS OUTLET, CLOSING THEVESSEL TO MAKE IT GAS-TIGHT, INTRODUCING AIR INTO THE GAS INLET TO BURNWITH THE CARBONACEOUS MATERIAL THEREBY PRODUCING THE HEAT TO RAISE THETEMPERATURE OF SAID ORE PELLETS AND ALSO RAISING THE PRESSURE ABOVEATMOSPHEREIC, EXHAUSTING THE COMBINATION PRODUCTS THROUGH SAID GASOUTLET TO LOWER THE PRESSURE, AND REPEATING THE CYCLE UNTIL ORE ISHEATED TO A FINAL TEMPERATURE ABOUT 1700* F.